As will be apparent to even the most casual observer, the brakes on a large tractor-trailer rig are of critical importance. This invention relates to a component of the brake system for the trailer or van on a fifth wheel tractor-trailer rig. The invention may also be used for trailers hauled by a drawbar or other coupling.
In a tractor-trailer the trailer air brakes are connected to the source of air pressure in the tractor by flexible hoses. Such a hose typically has one end secured to the back of the tractor. The other end of the hose has one moiety of a quick disconnect coupling of the type known in the trade as a "gladhand".
A second moiety of a gladhand coupling is secured to the front face of the trailer to receive the coupling on the hose end. The coupling on the front of the trailer is in turn connected to the trailer brakes.
A gladhand is a standard, commercially available connector for tractor-trailer air brake lines. Its features are specified in Society of Automotive Engineers (SAE) Specification J318, which is hereby incorporated by reference. A gladhand consists of two functionally identical moieties with threaded connection to an air hose or pipe. Each half of the coupling has a pneumatic pressure "inlet" port in one face. The inlet port has an elastomeric seal having an approximately flat annular face around the inlet port. It will be recognized that referring to this pneumatic pressure port as an "inlet" port is simply a matter of convenience for description herein. When the coupling is connected, the two moieties have their respective inlet ports in direct communication, and one therefore serves as inlet while the other serves as an outlet. It will also be noted that in practice of the invention as described herein, the inlet pressure port does serve as an inlet. In other embodiments it may be an outlet. Also, as a matter of convenience in this description, the face of the coupling having the seal is referred to as the "front" and the opposite face is referred to as the "back".
Each moiety of the gladhand has a wing at one end with an arcuate locking surface concentric with the inlet port. At the other end of the gladhand there is a pocket open at one end and having an arcuate locking surface concentric with the inlet port. To assemble the gladhand the rubber seals are placed in face to face engagement. One half of the coupling is then twisted relative to the other around the axis of the inlet port so that the wing on each half enters the open end of the pocket on the other half. The locking surfaces engage each other to hold the coupling in tight engagement. A short twist in the opposite direction disconnects the coupling.
When the brake pedal is depressed, air pressure in the brake lines builds up rapidly to apply the brakes. When the brake pedal is released, air pressure in the lines decreases and the brakes are released. Air pressure can be applied quickly since the tractor air pressure is typically about 95 psi (6.68 kg/cm.sub.2). The ON signal to apply the brakes travels quickly through the pneumatic tubing to the brake actuator. However, the response time is considerably longer for an OFF signal to release the brakes, since the entire service line must vent to about 5 psi (0.35 kg/cm.sub.2) before brake release will occur. For long rigs the release time may be from one to one and a half seconds.
There are several reasons why faster response to an OFF signal in truck air brakes is desired. Inexperienced or careless drivers may accelerate before the brakes fully release. In many situations a truck may continue to roll under its own momentum after the driver releases the brake pedal. In either event the trailer is dragged against some braking action that causes unnecessary wear on brake drums and shoes, and on tires. Further, slow release of the brakes can cause accidents, for example, in a situation where the road surface is slick, and skidding or sliding is a hazard.
It has been found that pressure exhaust or "dump" valves in truck air brake lines can reduce the trailer brake release time to about a half second. Pressure exhaust valves have been designed to promptly exhaust pressure from the air brake system when the air pressure commences decreasing and before the brakes release. These pressure exhaust valves have been placed in close proximity to the air brake actuators and reservoirs at the brake axles. In this location they may be 50 to 60 feet downstream from the tractor, and time is required for a brake release signal to reach and actuate such valves. It has now been found that optimum results are obtained when such a pressure exhaust valve is at the forward end of the trailer, preferably within 1.5 meters of the gladhand connecting the tractor air line to the trailer air line.
It is therefore desirable to provide a pressure exhaust valve at the forward end of a trailer for optimum brake operation. The location should be safe and readily accessible for maintenance.